Method for fastening an implant to bone tissue and corresponding implant system

ABSTRACT

An implant designed e.g. as a replacement for an articulating surface of a human or animal joint is secured to the bone tissue with the aid of a plurality of fasteners ( 3 ). The implant ( 2 ) comprises a bone side to be brought into contact with the bone tissue, which bone side is equipped with a plurality of fastening structures restricted to this bone side. The fasteners ( 3 ) comprise a distal and a proximal side, wherein the distal side is equipped for being anchored in bone tissue and the proximal side is equipped for being connected with one of the fastening structures of the implant ( 2 ). The distal sides of the fasteners ( 3 ) are anchored in the bone tissue of the appropriately prepared bone and the implant ( 2 ) is then attached to the anchored fasteners ( 3 ) by connecting the fastening structures with the proximal sides of the fasteners ( 3 ), wherein the implant ( 2 ) is pressed against the proximal sides of the anchored fasteners ( 3 ).

BACKGROUND OF THE INVENTION

The invention lies in the field of medical engineering and concerns amethod for fastening an implant to human or animal bone tissue as wellas a corresponding implant system, wherein the implant may be designede.g. as a replacement for the articulating surface of a joint, as areplacement for a larger bone section or another tissue or as an elementfor stabilizing a bone e.g. damaged by fracture. The invention furtherconcerns a kit for carrying out the method.

Replacements of articulating surfaces of human joints are known, inparticular, in connection with the tibia plateau in the knee joint(articulating surface with concave and convex portions), the glenoidcavity on the shoulder-blade and the acetabulum on the pelvis (concavearticulating surfaces), but are also used for articulating surfaces ofother joints (elbows, wrists, ankles or digital joints), in particularalso convex articulating surfaces such as e.g. the articulating surfaceof the femoral head.

Known implants replacing articulating surfaces of joints (resurfacingimplants) are flattish elements having a thickness which is smaller thantheir dimensions parallel to the bone surface and a concave and/orconvex form and they usually comprise an essentially centered shaft orcomb facing the bone tissue. These implants consist e.g. of a metal(e.g. titanium, titanium alloys or CoCr cast alloys) and carry on theirside, facing the joint a bearing element, constituting the actualarticulating surface and usually consisting of a synthetic material(e.g. polyethylene, in particular UHMWPE or ultra high molecular weightpolyethylene). According to the state of the art, the implant is securedin the cancellous bone tissue of the appropriately prepared bone bymeans of a bone cement, by means of inserting screws into the bonethrough corresponding bores in the implant and/or by means of the shaftor comb being malleted into a corresponding opening in the bone tissueto achieve a press fit. It is further suggested to secure resurfacingimplants not in the bone tissue beneath the implant, but in the corticalbone next to the implant, e.g. by clamping or screwing.

All named methods for securing resurfacing implants suffer specificdisadvantages. The use of bone cement does not allow significantosseointegration, which would be advantageous for a durable connection.Screws extending through the implant require through bores and thesebores and, subsequently, the heads of the screws inserted thereinconstitute friction points on the joint side of the implant, which isparticularly detrimental if the implant constitutes the articulatingsurface (without bearing element) and the bores therefore are part ofthe articulating surface. Such friction points may also reduce thelifespan of a bearing element if it is movable on the joint side of theimplant, as is usually the case for tibia plateau implants. Althoughsecurement by press fit permits osseointegration, it gives only alimited primary stability and, thus, necessitates a prolonged period ofrest for the repaired joint. The above mentioned lateral securingrequires additional space and therefore cannot be applied universally.

The implants for the replacement of larger bone parts or other tissuesmentioned above in addition to the resurfacing implants may e.g. replacea complete joint ball including the neck. Such implants usually comprisea lengthy shaft to be secured in the marrow space of a tubular bone,again with the aid of a cement or with the aid of a press fit. Accordingto the state of the art, implants for supporting or stabilizing bonese.g. damaged by fracture are often plates or rods with through bores tobe attached to the bone e.g. by means of screws. In many of these casesthe same aforementioned disadvantages of the named fastening methodsapply.

BRIEF SUMMARY OF THE INVENTION

For the reasons mentioned above an alternative way for fasteningimplants on human or animal bone tissue is desirable, wherein thisfastening does not, or does at least to a greatly reduced extent only,have the above mentioned disadvantages. Thus the object of the inventionis to create such an alternative fastening of implants on bone tissue,in other words, to create appropriately equipped implant systems and amethod for fastening these on appropriately prepared bone tissue.Therein the implant system according to the invention is to enableosseointegration, the implant side facing away from the bone tissue isto comprise as few structures serving the fastening function aspossible, and the fastening method is not to be more elaborate thanknown methods serving the same purpose.

In the following, the expression bone tissue does not only relate topurely native bone tissue but also to bone tissue reinforced byinjection of a cement based on calcium phosphate (osteoplasty, inparticular vertebroplasty), and to bone tissue augmented with the aid ofa bone replacement material.

The implant of the implant system according to the invention comprisesfastening structures (e.g. recesses and/or protrusions) on its onesurface which is to come into contact with the bone tissue. The implantsystem further comprises a plurality of fasteners, which, on a distalside, are equipped for being anchored in the bone tissue and, on aproximal side, for being connected with the fastening structures of theimplant. Each fastener is essentially assigned to one of the fasteningstructures and the fasteners are e.g. pin-shaped.

The implant according to the invention is fastened to the appropriatelyprepared bone tissue in two steps. In an anchoring step, the fastenersare anchored in the prepared bone. In a fastening step, the implant isattached to the proximal sides of the fasteners anchored in the bone, bymeans of the fastening structures, wherein the implant is pressedagainst the proximal sides of the fasteners. Therein the fastening stepis performed preferably after the anchoring step. For anchoring thefasteners, use of a corresponding template ensures that the positions ofthe anchored fasteners correspond with the positions of the fasteningstructures on the bone side of the implant. Advantageously, theanchoring of the fasteners is performed in sequence but the attachmentof the implant to all fasteners essentially simultaneously. In specificapplications of the method according to the invention it is alsopossible to execute only a part of the anchoring step (e.g. positioningof the fasteners in corresponding openings) prior to the fastening stepand subsequently effect the proper anchoring simultaneously with thefastening step.

In the method according to the invention, the connection to beestablished between the proximal sides of the fasteners and recesses orprotrusions of the implant serving as fastening structures may be apositive fit connection, a force fit connection and/or a materialconnection. The connection must be of such a design that it can beachieved from a side of the implant other than the implant side which isto be brought into contact with the bone tissue (in the case of flattishand accordingly accessible implants advantageously from the oppositeimplant side) and, if at all possible, simultaneously for all fasteners.Such a connection is produced e.g. by a snap action between two partscorrespondingly adapted to each other (positive fit connection) as knownfrom fastening dentures on tooth stumps or dental implants. A positivefit connection can also be produced by melting or at least softening athermoplastic material on one side and embedding this material inundercut recesses on the opposite side, wherein the energy required forliquefaction is supplied to the thermoplastic material through theimplant by heating suitable areas of the implant or by transmittingmechanical vibration to the implant. A material connection is achievede.g. by fusing thermoplastic material provided on both sides, whereinthe required energy is supplied to the thermoplastic material in thesame manner. For a force fit connection the fasteners comprise e.g.conic openings and the fastening structures of the implant consist oftapered protrusions adjusted to the conic openings in such a manner thatthey can be pressed into the openings and are, thus, held by aself-locking press fit. In all cases, the implant is advantageouslypressed against the proximal sides of the anchored fasteners with theaid of a correspondingly adapted pressing tool, which may be furtherequipped for transferring energy (e.g. heat or mechanical vibration) tothe implant.

For being anchored in the bone tissue, the fasteners preferably comprisea thermoplastic material on their distal side so that they can beanchored in the bone tissue by means of mechanical oscillation(advantageously ultrasonic vibration), as is known e.g. from thepublications U.S. Pat. Nos. 7,335,205, 7,008,226, WO-2005/079696, orWO-2008/034277. For such an anchoring it is advantageous, but notnecessary, to create, in the bone, openings which are adapted to thefasteners. Alternatively, the distal sides of the fasteners may bescrew-like, comprising e.g. self-tapping threads to be screwed into thebone tissue, wherein openings for accommodating the screws may or maynot be drilled into the bone beforehand. Fasteners with pointed distalsides and barbs are also conceivable, such fasteners being anchoredsimply by being malleted into the bone tissue. In principle, any knownanchoring methods or combinations thereof are suitable for anchoring thefasteners in the bone, in particular anchoring methods which aresuitable for anchorage in cancellous or even osteoporotic bone tissue,as is in particular the case for the first named anchoring method.Therein it is also possible to anchor the fasteners in correspondingopenings in the bone tissue with the aid of bone cement.

In most embodiments of the method according to the invention, the stepof anchoring the fasteners and the step of connecting the implant withthe anchored fasteners are essentially independent of each other and areexecuted after each other. Therefore, different embodiments of these twosteps as well as different embodiments of proximal and distal sides ofthe fasteners can be combined in an essentially freely selectablemanner. Particularly advantageous embodiments result from combinationsof two steps which can be carried out with simple fasteners and/or withfasteners whose height relative to the prepared bone surface afteranchoring can be adjusted by simple means so that only a limitedaccuracy is required for the anchoring step.

A kit for the execution of the method according to the inventioncomprises the implant system (implant and a plurality of appropriatefasteners) as well as at least one tool adapted to the implant and/or tothe fasteners. The tool is in particular a template enabling anchoringof the fasteners at the positions of the bone tissue corresponding withthe positions of the fastening structures on the implant and/or a toolfor pressing the implant against the proximal fastener sides.

If the implant, according to the invention, is a resurfacing implant ithas a flattish shape with a bone side to be brought into contact withthe bone tissue and a joint side opposite the bone side, i.e. facing thejoint. The joint side is equipped for mounting a bearing element thereonin a per se known manner e.g. by snapping the bearing element in beneathan appropriate implant rim or, in the case of a tibia plateau, on to apost which is movable to a limited degree in relation to the flatimplant part. On the other hand, the joint side of the resurfacingimplant may have a surface which is suitable as an articulating surface.Advantageously, the joint side of the implant does not comprise anystructures serving the securement of the implant to the bone, i.e. theproximal side of the fastener does not reach to a side of the implantfacing away from the bone side. These fastening structures are presenton the bone side of the implant, e.g. in the shape of a number ofrecesses and/or protrusions. Portions of the implant bone side betweenfastening structures which after implantation remain in close contactwith the bone tissue are preferably equipped in a per se known mannerfor enhancing osseointegration. Resurfacing implants according to theinvention can essentially be used for resurfacing all articulatingsurfaces of human and animal joints, in particular for the applicationsmentioned above in connection with the state of the art.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments and applications of method and implant systemaccording to the invention are described in more detail in connectionwith the following Figs., wherein:

FIG. 1A-1C illustrate a first exemplary embodiment of the methodaccording to the invention using the example of resurfacing a tibiaplateau, wherein the fasteners are anchored in openings provided in thebone tissue by means of a thermoplastic material and mechanicaloscillation and wherein the implant is connected in situ with thefasteners by a positive fit connection achieved by melting or softeninga thermoplastic material;

FIG. 2 shows a further exemplary embodiment of fastening structure andfastener, wherein a snap connection is provided between the two andwherein the fastener is anchored in the bone tissue by means of a screwthread;

FIG. 3 shows a further exemplary embodiment of fastening structure andfastener, wherein a force fit connection is provided between the two andwherein the fastener is anchored in the bone tissue by means of a cementor by means of a thermoplastic material and mechanical oscillation;

FIG. 4 shows a further exemplary embodiment of fastening structure andfastener, wherein a weld (material connection) is provided between thetwo and wherein the fastener is anchored in the bone tissue by beingmalleted into the bone tissue;

FIG. 5 shows a further embodiment of the method and a tool for fasteningthe implant to the fasteners, where implant and fasteners are the sameas illustrated in FIG. 1A;

FIGS. 6 and 7 show further embodiments of fasteners suitable for thefastening according to FIG. 2 or FIG. 3 respectively;

FIGS. 8 to 10 show further embodiments of method and implant systemaccording to the invention;

FIGS. 11 to 13 show further exemplary embodiments of fasteningstructures and fasteners suitable for the implant system according tothe invention;

FIGS. 14A to 14C show further exemplary fasteners and fastenerassemblies suitable for the implant system according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A, 1B and 1C illustrate a first exemplary embodiment of theimplant system and of the method for fastening the implant according tothe invention. These Figs. illustrate the resurfacing of a tibia plateau(articulating surface) as an exemplary application. As already mentionedabove, implant and method according to the invention are not onlysuitable for the resurfacing of a tibia plateau but essentially for anyreplacement of an articulating surface of a human or animal joint,wherein each specific application requires a corresponding adaptation ofthe implant regarding, in particular, shape and design of the joint sideand a corresponding adaptation of the fasteners regarding number,dimensions and positions in relation to the bone and the implant. Thenamed adaptations can be carried out by one skilled in the art withoutany problem. Implant and method as illustrated in FIGS. 1A to 1C arealso suitable for supporting or stabilizing bones (e.g. after afracture), wherein in such a case the implant is fashioned as plate orrod and is usually secured in cortical bone tissue with the aid of thefasteners.

FIG. 1A shows in section the tibia bone 1 prepared for resurfacing in agenerally known manner, the implant 2 having a generally flat form, anda plurality of fasteners 3, of which e.g. four are required for securingthe tibia plateau implant (FIG. 1C shows exemplary positions of thefasteners on the tibia plateau which is viewed from the top). A bearingelement 4 to be locked into the implant on the joint side thereof isalso shown.

The implant 2 consists in a generally known fashion e.g. of a metal(e.g. titanium, titanium alloy, CoCr cast alloy), of a ceramic material(e.g. aluminum oxide or zirconium oxide), of a composite material (e.g.filled PEEK) or of a high strength plastic material without filler(preferably cristalline polymer having a glass transition temperatureabove 100° C. or thermosetting plastic). The fastening structuresprovided on the bone side of the implant 2 are in the present caseundercut recesses 5, their positions determining the anchoring positionsof the fasteners 3.

In the embodiment according to FIG. 1A, the fasteners 3 are simple pinsof a medically acceptable material with thermoplastic properties. Theirproximal ends are adapted to the entrance of the recesses 5 and theirdistal ends are e.g. pointed or tapering.

FIG. 1B illustrates four consecutive phases of the method according tothe invention, wherein the implant system according to FIG. 1A isattached to the end of the tibia bone 1 facing the knee-joint.

First, openings 20 for anchoring the fasteners 3 in the bone tissue 1are produced, advantageously using a template 10 and a drilling tool 11.The shape of the template 10 corresponds (in particular on the boneside) with the implant 2 and comprises bores 12 in positions (FIG. 1C)corresponding with the positions of the recesses 5 in the implant, thediameter of which bores is adapted to the diameter of the drilling tool11. The template 10 is secured on the prepared bone surface withsuitable means and the openings 20 are made by moving the drilling tool11 through the bores 12. Advantageously, the drilling tool 11 comprisesgenerally known means for setting a required depth for the openings 20.

When the openings 20 are made, the template 10 is removed and one afterthe other of the fasteners 3 is anchored in the openings, wherein eachone the fasteners are positioned in one of the openings and is thenpressed into the opening 20 while simultaneously being vibrated with theaid of an oscillating tool 21 (e.g. sonotrode of an ultra-sonic devicewith a distal coupling surface adapted to the proximal face of thefastener 3). The friction between the surface of the fastener 3 and thebone tissue at the bottom and/or wall of the opening 20 caused by theoscillation leads to the thermoplastic material of the fastener 3 beingliquefied and being pressed into the bone tissue in liquid form, whilethe fastener 3 is simultaneously advanced in the opening 20. When thefastener 3 is sufficiently advanced in the opening 20, i.e. when asufficient amount of thermoplastic material has been pressed into thebone tissue, the oscillation is stopped so that the liquefied materialpenetrating the bone tissue re-solidifies and the fastener 3 is, thus,anchored in the bone tissue essentially by a positive fit connection.FIG. 1C shows four fasteners 3 anchored in the bone tissue.

Once all the fasteners 3 are anchored, the implant 2 is positioned onthe fasteners 3 in such a way that the proximal sides of the fasteners 3which protrude from the bone tissue, are positioned in the recesses 3.The implant 2 is then pressed towards the bone surface with the aid of apressing tool 30 the face of which is adapted to the joint side of theimplant 2, while heat is supplied in a suitable manner (e.g. via heatconductor or heating elements 31) to the implant areas correspondingwith the fastener positions. The proximal ends of the fasteners 3 arepressed into the recesses 5 by the pressure from the pressing tool 30being pressed against the implant 2, or against the fasteners 3respectively. The thermoplastic material of the fasteners yields throughthe contact with the locally heated implant and adapts in form to theundercut shape of the recesses 5, thus creating the desired positive fitconnection between implant 2 and fastener 3.

The completed securement of the implant 2 in the bone 1 is shown at thebottom of FIG. 1B. Preferably, in particular for a resurfacing implant,the length of the fasteners 3 is such that the bone side of the implant2, as illustrated at the bottom in FIG. 1B, touches the bone tissuebetween the fasteners 3. This enables osseointegration of the implantwhose surface facing the bone is pre-treated accordingly (e.g. roughenedsurface of titanium or titanium alloy or a surface carrying a coatingbased on calcium phosphate).

Suitable materials with thermoplastic properties for the fasteners 3according to FIGS. 1A and 1B are e.g.: resorbable polymers such aspolymers based on lactic and/or glycolic acid (PLA, PLLA, PGA, PLGAetc.) or polyhydroxy alkanoates (PHA), polycaprolactone (PCL),polysaccharides, polydioxanes (PD) polyanhydrides, polypeptides orcorresponding copolymers or composite materials containing the namedpolymers as a component; or non-resorbable polymers such as polyolefines(e.g. polyethylene), polyacrylates, polymetacrylates, polycarbonates,polyamides, polyester, polyurethanes, polysulfones, polyarylketones,polyimides, polyphenylsulfides or liquid crystal polymers LCPs,polyacetales, halogenated polymers, in particular halogenatedpolyolefines, polyphenylensulfides, polysulfones, polyethers orequivalent copolymers or composite materials containing the namedpolymers as a component.

Specific embodiments of degradable materials are Polylactides like LR706PLDLLA 70/30, R208 PLDLA 50/50, L210S, and PLLA 100% L, all ofBöhringer. A list of suitable degradable polymer materials can also befound in: Erich Wintermantel und Suk-Woo Haa, “Medizinaltechnik mitbiokompatiblen Materialien und Verfahren”, 3. Auflage, Springer, Berlin2002 (in the following referred to as “Wintermantel”), page 200; forinformation on PGA and PLA see pages 202 ff., on PCL see page 207, onPHB/PHV copolymers page 206; on polydioxanone PDS page 209. Discussionof a further bioresorbable material can for example be found in CABailey et al., J Hand Surg [Br] 2006 April; 31 (2):208-12.

Specific embodiments of non-degradable materials are: Polyetherketone(PEEK Optima, Grades 450 and 150, Invibio Ltd), Polyetherimide,Polyamide 12, Polyamide 11, Polyamide 6, Polyamide 66, Polycarbonate,Polymethylmethacrylate, Polyoxymethylene. An overview table of polymersand applications is listed in Wintermantel, page 150; specific examplescan be found in Wintermantel page 161 ff. (PE, Hostalen Gur 812, HöchstAG), pages 164 ff. (PET) 169 ff. (PA, namely PA 6 and PA 66), 171 ff.(PTFE), 173 ff. (PMMA), 180 (PUR, see table), 186 ff. (PEEK), 189 ff.(PSU), 191 ff (POM-Polyacetal, tradenames Delrin, Tenac, has also beenused in endoprostheses by Protec)

The thermoplastic materials may contain foreign phases or compoundsserving further functions. In particular, the thermoplastic material maybe strengthened by admixed fibers or whiskers (e.g. of calcium phosphateceramics or glasses) and such represent a composite material. Thethermoplastic material may further contain components which expand ordissolve (create pores) in situ (e.g. polyesters, polysaccharides,hydrogels, sodium phosphates) or compounds to be released in situ andhaving a therapeutic effect, e.g. promotion of healing and regeneration,e.g. growth factors, antibiotics, or inflammation inhibitors.Furthermore, buffers such as sodium or calcium phosphate or calciumcarbonate may be contained in resorbable thermoplastic materials againstadverse effects of acidic decomposition as described e.g. in Heidemannet. al. “pH-stabilization of predegraded PDLLA by an admixture ofwatersoluble sodiumhydrogenphosphate”, Biomaterials 2002, September;23(17):3567-74. If the thermoplastic material is resorbable, release ofsuch compounds is delayed.

Fillers used may include degradable, osseostimulative fillers to be usedin degradable polymers, including: β-Tricalciumphosphate (TCP),Hydroxyapatite (HA, <90% crystallinity; or mixtures of TCP, HA, DHCP,Bioglasses (see Wintermantel).

Osseointegration stimulating fillers that are only partially or hardlydegradable, for non degradable polymers include: Bioglasses,Hydroxyapatite (>90% cristyllinity), HAPEX®, see SM Rea et al., J MaterSci Mater Med. 2004 September; 15(9):997-1005; for hydroxyapatite seealso L. Fang et al., Biomaterials 2006 July; 27(20):3701-7, M. Huang etal., J Mater Sci Mater Med 2003 July; 14(7):655-60, and W. Bonfield andE. Tanner, Materials World 1997 January; 5 no. 1:18-20.

Embodiments of bioactive fillers and their discussion can for example befound in X. Huang and X. Miao, J Biomater App. 2007 April; 21(4):351-74), JA Juhasz et al. Biomaterials, 2004 March; 25(6):949-55.

Particulate filler types include: coarse type: 5-20 μm (contents,preferentially 10-25% by volume), sub-micron (nanofillers as fromprecipitation, preferentially plate like aspect ratio>10, 10-50 nm,contents 0.5 to 5% by volume).

For the fasteners being anchored as discussed above, it is advantageousfor the thermoplastic material to have a modulus of elasticity of morethan 0.5 GPa, i.e. to be able to transmit the mechanical vibration whichusually has a frequency in the range of 2 to 200 kHz with little dampingfrom the proximal face to the surfaces which are in contact with thebone tissue. For keeping the thermal load within an acceptable range,the material liquefies at a temperature of less than about 350° C. Forinitiating and/or facilitating liquefaction in the named contact areasit may be advantageous to provide in these contact areas a preferablythin contact layer of a “softer” material which dampens the mechanicalvibration more and therefore is more easily heated and therewithliquefied. Suitable measures and materials for providing such contactlayers are described in the co-pending U.S. application Ser. No.60/888,798, which is incorporated herein by reference. For thermoplasticmaterial of a high cristallinity such as e.g. polyether aryleketones(PEEK or PEAK) or PLLA anchorage in the above described manner is hardlypossible without the named contact layer.

The embodiment of the invention as shown in FIGS. 1A and 1B comprisesthe following advantages:

-   -   The anchoring by means of thermoplastic material and mechanical        oscillation is suitable in particular for anchorage in bone        tissue with little stability (cancellous or osteoporotic bone        tissue).    -   The form requirements for the proximal side of the fasteners 3        are minimal (they must be capable of being inserted into the        recesses 5), so that these sides can be trimmed subsequent to        anchoring in order to compensate for differing anchoring depths        of individual fasteners 3 due to differing bone qualities. Such        trimming is very simple as a superfluous length of a fastener        can e.g. simply be cut off.    -   The recesses 5 being designed to be at least slightly larger        than the proximal ends of the fasteners allow at least to a        modest degree for an adjustment of the implant's lateral        position subsequent to the anchoring of the fasteners.    -   The fasteners 3 may comprise any chosen, in particular        non-circular cross-section that can be adapted to the osseous        conditions and/or to the load distribution over the articulating        surface to be resurfaced.    -   The fasteners 3 may fulfill further functions in addition to the        implant securement, in particular reinforcement of bone tissue        with little stability, repair of lacerations in the bone tissue,        or fastening of bone fragments or augmentation materials.

The implant system as well as the method according to FIGS. 1A to 1C canbe modified in widely differing ways. Some examples follow:

-   -   The recesses 5 are situated in protrusions of the bone side of        the implant and the entrances of the openings 20 comprise a        corresponding width (similar to the embodiment according to        FIG. 2) such that the protrusions can be positioned therein.    -   The fasteners 3 do not consist entirely of the thermoplastic        material but comprise a core of metal, of ceramic, of a        thermoplastic material which does not liquefy under the        implantation conditions, or of a thermosetting plastic material,        wherein the surface of this core is completely or just partly        covered with the liquefiable thermoplastic material (for an        example see FIGS. 6 and 7). The fasteners may also comprise a        non-liquefiable sheath in which the liquefiable material is        placed, and from which it is squeezed out through appropriate        openings in the sheath and into the bone tissue when the        vibration is applied and the material is therewith liquefied. As        a liquefiable material, a thermoplastic polymer or a polymer,        ceramic or hydraulic cement with thixotropic properties (e.g.        Norian® of Synthes or Sulfix® of Centerpulse) may be provided in        the sheath (for an example see FIGS. 11 and 13).    -   The core or sheath mentioned above may comprise surface areas        remaining free of the liquefiable material and being equipped        for enhancing osseointegration and/or comprising self-cutting        edges as disclosed in the publications U.S. Pat. No. 7,008,226        and WO-2005/079696 which are enclosed herein by reference.    -   The core or sheath of the fastener comprises the undercut cavity        and the implant part comprises protrusions of the thermoplastic        material serving as fastening structures.    -   The thermoplastic materials of distal and proximal sides of the        fasteners 3 are different.    -   The fasteners 3 are pins with a cross-section other than round,        so that a means other than the above described drilling tool 11        is to be provided for producing the openings 20.    -   The fasteners 3 are not pin-shaped but have the shape of tongues        or blades extending e.g. parallel to the rim of the implant 2 as        illustrated in FIG. 1C in dash-dot lines and denominated with        3′.    -   Fasteners 3 of different shapes and/or dimensions are provided        for positions of different load levels upon the articulating        surface.    -   The fasteners 3 are not anchored in the openings 20 by means of        mechanical oscillation but with the aid of a curable bone        cement.    -   The fasteners 3 are slim and pointed and they are anchored in        the bone tissue with the aid of mechanical oscillation and        pressure without providing openings 20 (or providing only        partial openings, e.g. only through a cortical bone layer).    -   The implant may also comprise, in a per se known fashion, a        shaft 7 (indicated by a dash-dot-line in FIG. 1A) or comb on its        bone side to be positioned in a corresponding shaft opening 7′        situated in the bone tissue. Such a shaft or comb, provided        according to state-of-the-art technology for the absorption of        shearing stress is however not necessary according to the        invention as such shearing stress is easily absorbed by the        fasteners 3.    -   Instead of the heat supply, mechanical oscillation may be        coupled into the implant for heating the proximal ends of the        implant's joint side. To this end e.g. an oscillating tool 32 is        used as shown in FIG. 5. This tool is e.g. part of an ultrasonic        device and comprises a distal end with oscillating extensions        33, the positions of which are adapted to the positions of the        fasteners.    -   The fasteners 3 are anchored in openings with the aid of a        thermoplastic material and mechanical oscillation, wherein the        proximal side of the fastener is kept at a predetermined        position relative to the bone surface. An example of such a        anchoring procedure is illustrated in FIG. 12. Further        embodiments are described in the co-pending U.S. application        Ser. No. 60/983,791, which is enclosed herein by reference.    -   The fasteners are only positioned in the openings 20 prior to        the fastening step, wherein anchoring of the fasteners and        fastening of the implant to the proximal fastener sides is        achieved simultaneously by coupling mechanical vibration into        the implant (see FIG. 13).

FIG. 2 shows a further exemplary embodiment of a connection between animplant 2 and a fastener 3 suitable for the implant system according tothe invention and of a suitable anchoring of the fastener 3 in the bonetissue 1, wherein just a section of the implant 2 and a single fastener3 are shown in an axial section and wherein the implant is fashionedonce more as a flattish or flat implant e.g. a resurfacing implant. Theleft-hand side of FIG. 2 illustrates the fastener 3 already anchored andthe implant 2 prior to being attached to the fastener. The right-handside illustrates the implant after the fastening step.

Again, the implant 2 comprises, as fastening structures, undercutrecesses 5 which are situated e.g. in protrusions 6. The fastener 3comprises a screw-shaped distal side and a proximal head 40 equipped forbeing coupled to a screw driving tool (e.g. comprising an outer or innerhexagon or a slot) and being adapted to the recess 5. Protrusion 6and/or head 40 are sufficiently elastic to allow for the head 40 to snapinto the recess 5. In order to establish contact for osseointegrationbetween the surface of the bone side of the implant 2 and the bonesurface, the opening to be provided in the bone tissue 1 for receivingthe fastener 3 comprises an entrance 41 which is adapted to theprotrusion 6. From this entrance 41, a bore may be provided for thescrew or the screw may be screwed into the bone tissue 1 without priordrilling.

Instead of the protrusion 6 with recess 5, a head constituting thefastening structure may be provided on the implant, whilst the fasteneris equipped with an undercut recess adapted to this head. Instead of thedistal side of the fastener being designed as a screw, it may also, atleast in part, comprise a surface of a material with thermoplasticproperties suitable for anchorage with the aid of ultrasonic vibration.An example of such a fastener is illustrated in FIG. 6.

Similar to FIG. 2, FIG. 3 shows a further connection between implant 2and fastener 3 suitable for the implant system according to theinvention and a suitable anchoring of the fastener 3 in the bone tissue1. The implant is again of a flattish shape and therefore is e.g.suitable for resurfacing the articulating surface of a joint.

Again, the fastener 3 consists e.g. at least in part of a thermoplasticmaterial liquefiable by mechanical oscillation and is anchored in acorresponding opening 20 in the bone tissue 1 by means of mechanicaloscillation, as described for the embodiment according to FIGS. 1A to1C. The fastener comprises a conical opening 50 adapted to acorresponding and equally conical protrusion 6 on the bone side of theimplant 2, so that the protrusion 6 can be pressed into the opening 50to be retained therein by a force fit connection (press fit). Suchpressing-in can indeed be all that is required for the securing step. Ifapplicable, the connection between the walls of the opening 50 and theprotrusion 6 may be reinforced by ultrasonic welding. In such a case theconic protrusion 6 or at least the surface thereof must also consist ofa thermoplastic material, which is capable of being fused to thethermoplastic material of the fastener 3. Such welding can be achievedby applying an oscillation tool 21 on the joint side of the implant 2opposite the protrusion 6, as indicated on the right-hand side of FIG.3. It is also possible to coat just one side (cone 6 or conical 50opening) with a thermoplastic material and equip the opposite side withsurface structures to be penetrated by the thermoplastic material whenit is liquefied by oscillation, which results in a positive fitconnection.

A similar connection between implant 2 and fastener 3 as illustrated inFIG. 3 is achieved if the conical opening is situated on the implant andthe cone on the fastener 3. Such a fastener 3 is illustrated in FIG. 7.The cone 51 and a core 52 extending to the distal side of the fastener 3consist e.g. of a metal and the core 52 is at least partly surrounded bythe thermoplastic material 53.

FIG. 4 shows a further connection between implant 2 and fastener 3 whichis suitable for the method according to the invention and the anchoringof the fastener 3 in the bone tissue 1, again assuming a flattishimplant e.g. a resurfacing implant.

The fastener 3 comprises at least on its proximal side a material withthermoplastic properties and is shaped like an anchor equipped with asharp point and cutting edges, and possibly with structures acting asbarbs. The fastener 3 is anchored by being malleted into the bone tissue1. As fastening structures on its bone side, the implant 2 compriseslocations 60 consisting of a material which is capable of being fusedwith the thermoplastic material of the fastener 3. Advantageously, theproximal side of the fastener 3 and the locations 60 are designed forself-centering so that they mesh at least slightly (e.g. slightlyconcave and convex respectively, as illustrated). For connecting theimplant 2 with the fastener 3, an oscillating tool 21 (e.g. sonotrode ofan ultrasonic device) is applied to the joint side of the implant 2,wherein the positions of the locations 60 are advantageously markedaccordingly.

The fastener 3 according to FIG. 4 may be pin-shaped, but may, inparticular, also be shaped as a blade (see FIG. 14A).

FIG. 8 illustrates, highly schematically, a further exemplary embodimentof the implant system according to the invention. It serves forresurfacing a femoral head (convex articulating surface). Allembodiments of the invention illustrated in the previous FIGS. 1 to 7can also be applied to this case. Similarly, implants having the shapeof plates or rods can be secured to a bone, the plates or rods havingsupporting and stabilizing functions e.g. in the case of bone fractures.In contrast to the resurfacing implants which are attached to mainlycancellous bone tissue, the named plates and rods are usually fixed tocortical bone tissue and usually no osseointegration is desired on thebone side of the implant.

The implants hitherto described are of a flattish shape and the implantside facing the bone tissue is essentially accessible for theapplication of tools. However, the method according to the invention isalso applicable when this is not the case. Examples of such applicationsare intervertebral disc implants to be introduced between two vertebraeand to be secured to at least one of them or implant shafts to besecured in a tubular bone such as e.g. the shaft of a hip jointprosthesis. The same may apply for a joint resurfacing implant, inparticular for a tibia plateau implant, if the implant is introducedinto the knee joint without making the tibia end fully accessible.

FIGS. 9 and 10 illustrate two embodiments of the implant systemaccording to the invention which are suitable for the above namedapplications, in particular if access to the implantation site islimited, such that the implant needs to be moved into its implantedposition in a direction which is not perpendicular but substantiallyparallel to the bone surface to which the implant is to be fastened andsuch that access to the side opposite the bone side of the implant islimited or not possible. The procedure of anchoring the fasteners 3 andthe means and ways for establishing the connection between the fastenersand the fastening structures of the implant are substantially the sameas described above and therefore need not be described in detail again.

FIG. 9 illustrates a vertebral disc implant 2. The fasteners 3 areanchored in the upper (and/or lower) side of a vertebral body 70. Forpositioning and securing the implant 2 between the two neighboringvertebrae, it is pushed (pressing tool 30, with which heat or mechanicaloscillation may also be transmitted) perpendicularly to the axes of thefasteners 3 against the lateral area of the proximal sides of thefasteners 3 which thereby are connected with the lateral walls of therecesses in the implant. For facilitating the pushing, the recesses inthe implant are advantageously laterally staggered and groove-like openat the leading side of the implant (not illustrated).

If the principle illustrated in FIG. 9 is applied to a prosthesis shaft,the bone tissue represented below and above the implant would be thewalls of the tubular bone in which the shaft is to be secured, andfasteners would preferably be provided on all sides. If the principleillustrated in FIG. 9 is applied to a tibia plateau implant, the bonetissue represented below the implant is the knee end of the tibia andthe tissue represented above the implant is the knee end of the femur orthe meniscus respectively. In all named cases the implant side oppositeits bone side is not or not easily accessible for applying the pressureand for coupling-in the energy needed for the fastening step.

For anchoring fasteners with the aid of a liquefiable material andmechanical vibration in implantation sites of a limited accessibility asshown in FIGS. 9 and 10, it is advantageous to use a vibrating tool ableto redirect the oscillation direction. Such a tool is e.g. described inthe publication WO-2007/101362.

FIG. 10 illustrates a further embodiment of the application similar tothe one according to FIG. 9, wherein fasteners 3 are not anchored in adirection which is substantially perpendicular to the bone surface butoblique. This embodiment is particularly suitable for securing aprosthesis shaft in a tubular bone but is also suitable e.g. asmentioned above for other implants such as e.g. an intervertebral diskimplant or a tibia plateau implant.

FIG. 11 illustrates the already further above mentioned fastener 3comprising a sheath 80 and, positioned within the sheath, theliquefiable material which on application of a vibrating tool to theproximal end thereof is liquefied at least partly and is squeezed out ofopenings provided in the sheath. FIG. 11 shows on the left hand side thefastener which is positioned for being anchored and, above the fastener,a fastening structure of the implant 2 suitable for being fastened tothe proximal end of the sheath. On the right hand side, fastener andimplant are shown after anchorage of the fastener and securement of theimplant on the proximal fastener side.

An opening 20 is provided in the bone tissue and the fastener ispositioned therein. If it is desired to prevent the bottom of theopening 20 to be damaged by pressing the sheath into the opening 20, itis suggested to equip the proximal end of the sheath 80 with a flange 81of a diameter larger than the diameter of the opening 20 and to choosean opening depth which is at least as great as the axial length of thesheath 80. The fastener is anchored in the opening 20 by pressing avibrating tool (not illustrated) against the proximal end of theliquefiable material and therewith at least partly liquefy the materialand pressing it out of the sheath openings into the bone tissue.

The fastening structure of implant 2 comprises a protrusion 82 of athermoplastic material or being coated therewith, which protrusion has across section being slightly larger than the cross section of the mouthregion of the sheath. On the inside of this mouth region e.g. a thread83 or other suitable structure is provided. When the implant 2 ispressed against the proximal side of the sheath and is vibratedsimultaneously, the protrusion 82 is forced into the mouth of the sheath80 whereby the thermoplastic material of the protrusion is liquefiedwhere it gets into contact with the thread 83, is pressed into thethread and forms therewith a positive fit connection onre-solidification.

It may be advantageous to equip the outside surface of the sheath 80with a surface which is able to do further osseointegration as disclosedin the publication U.S. Pat. No. 7,008,226 and/or with self cuttingedges as disclosed in the publication WO-2005/079696.

FIG. 12 shows an example of a fastener which can be anchored in bonetissue, wherein the proximal fastener end maintains a predeterminedposition relative to the bone surface, e.g. as illustrated: flush withthe bone surface. Again, the fastener 3 is shown on the left hand sidewhen positioned in a corresponding opening 20 for being anchoredtherein, and on the right side in an anchored configuration with theimplant fastened to its proximal side.

The fastener 3 comprises two parts: a tube 85 of a thermoplasticmaterial and a foot piece 86 which may also consist of a thermoplasticmaterial. The foot piece is connected to the distal end of the vibratingtool 87, e.g. screwed thereon, wherein the vibrating tool extendsthrough the tube 85 and through a tube shaped counter element 88. Forthe anchoring procedure, the vibrating tool 87 with the counter element88, the tube 85, and the foot piece 86 mounted thereon, is positioned inthe opening 20, wherein the counter element 88 is fixed in a positionsuch that its distal end is in the position which is predetermined to bethe position of the proximal fastener end after anchorage. The vibratingtool is then pulled in a direction away from the opening 20 and vibratedsuch that the foot piece 86 pushes the tube 85 against the counterelement 88 whereby the material of the tube is liquefied where incontact with the foot piece 86 and penetrates the bone tissue at leastin the vicinity, and whereby the tube may be fused to the foot piece.Through the vibration, the foot piece (if consisting of a thermoplasticmaterial) gets warm and therewith soft such that eventually thevibrating tool can be pulled from the foot piece 86, which remains as apart of the anchored fastener in the bone tissue. If the foot piece 86does not consist of a thermoplastic material, the vibrating tool 87 can,after the anchoring procedure, be separated from the foot by screwing itout or it may be left in place for a proximal end thereof serving forthe fastening of the implant.

As shown on the right hand side of FIG. 12, a fastening structure of theimplant 2 suitable for being connected with the fastener is e.g. anundercut protrusion 82 of a material which is not liquefiable, theprotrusion 82 being forced into the anchored fastener while beingvibrated.

Further embodiments of fasteners and anchoring procedures similar to theone illustrated in FIG. 12 are disclosed in the co-pending applicationUS 60/983,791, examples of implantation systems suitable for positioningand anchoring the fastener are disclosed in co-pending application US61/033,066.

FIG. 13 illustrates a fastener 3 and fastening structures of an implant2 which are suitable for first positioning the fastener 3 in a boneopening 20, then positioning the implant 2 on the proximal end of thefastener and then applying pressure and vibration to the fastener andtherewith anchor the fastener and fasten the implant to the fasteneressentially simultaneously. The fastener again comprises a sheath 80 anda liquefiable material therein. The implant 2, comprises as fasteningstructure, a protrusion with a distal undercut cavity, the cross sectionof the protrusion being adapted to the inner cross section of the sheath80. On pressing the implant against the fastener being positioned in theopening 20 and applying mechanical vibration to the implant, thevibrating protrusion 82 is pressed against the liquefiable materialinside the sheath, thereby at least partly liquefying it and pressing itout of the sheath into the bone tissue on the one hand and into thecavity of the protrusion on the other hand to form in both places apositive fit connection on resolidification.

FIGS. 14A to 14C show examples of further fasteners and fastenerassemblies which are applicable in the implant system according to theinvention.

FIG. 14A shows a blade-shaped fastener. As illustrated in FIG. 1C, sucha fastener (3′) may replace two, or more than two, pin-shaped fastenersand, if sufficiently long and appropriately shaped, can take over in thesense of a fastener assembly the function of the plurality of fastenersas described above. Such a longish blade-shaped or also tongue-shapedfastener has preferably a curved or folded form (zig-zag or wavy form)or is e.g. closed to form a circle or oval.

The blade shaped fastener illustrated in FIG. 14A having a sharp distalside is suitable for being malleted into the bone tissue. The proximalside comprises a coating of a thermoplastic material and is suitable tobe fastened to an implant as shown in FIGS. 1A to 1C. Obviously allother combinations of fastening structures and proximal fastener sidesare applicable also for a blade-or tongue-shaped fastener.

FIGS. 14B and 14C show an assembly of a pin-shaped fastener with aplurality of blade-shaped fasteners, wherein the blade shaped fastenersextend radially from a proximal portion of the pin-shaped fastener (FIG.14B: viewed from the proximal side; FIG. 14C: viewed perpendicular tothe pin axis). While the pin-shaped fastener is e.g. made of thethermoplastic material, the blade-shaped fasteners are made of metal andhave a sharp distal end and polymer coated proximal heads of a form asshown for the blade-shaped fastener of FIG. 14A. For anchoring thefastener assembly in the bone tissue, an opening corresponding to thepin-shaped fastener is provided and while the pin-shaped fastener isintroduced into the opening on being vibrated and therewith anchored,the blade-shaped fasteners cut into the bone tissue to be anchoredtherein by a press fit. Therein the blade-shaped fasteners are guided bythe pin-shaped fastener and the opening in the bone tissue respectively,which guiding constitutes an advantage of the fastener assemblyaccording to FIGS. 14B and 14C as compared with the blade-shapedfastener as shown in FIG. 14A.

In particular for resurfacing implants, where osseointegration of thebone side of the implant is highly desirable, it is important to leaveas much of the bone side of the implant in direct contact with the bonetissue and to provide in the bone as little and as small openings aspossible, but still to achieve a sufficiently strong fastening of theimplant on the bone. For such applications it may be advantageous to usefastener assemblies as shown in FIGS. 14A to 14C.

As already mentioned further above, it is possible to combine anchoringprocedures of the fasteners and connecting procedures between thefasteners and the fastening structures of the implant other thanillustrated in the above described FIGS. Similarly, it is obvious forone skilled in the art that other properties of the embodimentsillustrated in the FIGS. can be combined in ways different from thecombinations which are described explicitly above. Such alternativecombinations are also part of the invention.

1. Method for fastening an implant to human or animal bone tissue), the method comprising the steps of: preparing the bone tissue; providing an implant comprising a bone side to be brought into contact with the bone tissue, the bone side of the implant being equipped with a plurality of fastening structures restricted to said bone side, providing a plurality of fasteners, each with a distal side and a proximal side, wherein the distal side is equipped for anchoring the fastener in the bone tissue and the proximal side is equipped for a connection with the fastening structures of the implant so that the proximal side of the fastener does not reach to a side of the implant facing away from the bone side, and anchoring the distal sides of the fasteners in the prepared bone tissue prior to fastening the implant to the fasteners by connecting the fastening structures with the proximal sides of the fasteners, wherein the implant is pressed against the proximal sides of the fasteners, wherein at least one of the following conditions holds: one of: the fastening structures and the proximal sides of the fasteners, comprises undercut recesses and the other one of: the fastening structures and the proximal sides of the fasteners, comprises protrusions which are insertable or snappable into the undercut recesses; one of: the fastening structures and the proximal sides of the fasteners, comprises undercut recesses and the other one of: the fastening structures and the proximal sides of the fasteners, comprises a thermoplastic material embeddable in the undercut recesses when melted or at least softened; one of: the fastening structures and the proximal sides of the fasteners, comprises conical protrusions and the other of: the fastening structures and the proximal sides of the fasteners, comprises conical recesses, wherein pressing the conical protrusions into the conical recesses results in a press fit connection; the fastening structures and the proximal sides of the fasteners comprise thermoplastic material capable of making a material connection wherein, for the step of fastening, heat or mechanical oscillation is applied to the implant from a side other than the bone side.
 2. Method according to claim 1, wherein all fastening structures are connected to the proximal sides of the fasteners simultaneously.
 3. Method according to claim 1, wherein the fastening structures are connected with the proximal sides of the fasteners by positive fit connections achieved by liquefying or softening and re-solidifying a thermoplastic material, by snap connections, by press fit connections and/or by fusing two thermoplastic materials together.
 4. Method according to claim 1, wherein the fasteners are anchored in openings provided in the bone tissue.
 5. Method according to claim 1, wherein the fasteners are anchored in the bone tissue by means of mechanical oscillation and a material which is liquefiable by mechanical oscillation, by means of a screw thread, by means of malleting the fasteners into the bone tissue and/or by means of a bone cement.
 6. Method for fastening an implant to human or animal bone tissue), the method comprising the steps of: preparing the bone tissue; providing an implant comprising a bone side to be brought into contact with the bone tissue, the bone side of the implant being equipped with a plurality of fastening structures restricted to said bone side, providing a plurality of fasteners, each with a distal side and a proximal side, wherein the distal side is equipped for anchoring the fastener in the bone tissue and the proximal side is equipped for a connection with the fastening structures of the implant so that the proximal side of the fastener does not reach to a side of the implant facing away from the bone side, and anchoring the distal sides of the fasteners in the prepared bone tissue prior to fastening the implant to the fasteners by connecting the fastening structures with the proximal sides of the fasteners, wherein the implant is pressed against the proximal sides of the fasteners, wherein at least one of the following conditions holds: one of: the fastening structures and the proximal sides of the fasteners, comprises undercut recesses and the other one of: the fastening structures and the proximal sides of the fasteners, comprises protrusions which are insertable or snappable into the undercut recesses; one of: the fastening structures and the proximal sides of the fasteners, comprises undercut recesses and the other one of: the fastening structures and the proximal sides of the fasteners, comprises a thermoplastic material embeddable in the undercut recesses when melted or at least softened; one of: the fastening structures and the proximal sides of the fasteners, comprises conical protrusions and the other of: the fastening structures and the proximal sides of the fasteners, comprises conical recesses, wherein pressing the conical protrusions into the conical recesses results in a press fit connection; the fastening structures and the proximal sides of the fasteners comprise thermoplastic material capable of making a material connection wherein the step of anchoring comprises pushing the fastener into an opening provided in the bone tissue and wherein the proximal side of the anchored fastener is trimmed before the step of fastening. 